Influence of o-phase on mechanical properties and corrosion resistance of duplex stainless steels

Influence of o-phase on mechanical properties and corrosion resistance of duplex stainless steels

\ PERGAMON Corrosion Science 30 "0888# 0504Ð0520 In~uence of s!phase on mechanical properties and corrosion resistance of duplex stainless steels N[...

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\ PERGAMON

Corrosion Science 30 "0888# 0504Ð0520

In~uence of s!phase on mechanical properties and corrosion resistance of duplex stainless steels N[ Lopez\ M[ Cid\ M[ Puiggali Universite Bordeaux I\ Laboratoire de Mecanique Physique URA CNRS 756\ Cours de la Liberation\ 22394\ Talence\ France Received 11 October 0886^ accepted 05 October 0887

Abstract In order to appraise the in~uence of s!phase on the behaviour of duplex stainless steels\ two tests are performed[ The _rst one is the double loop electrochemical potentiodynamic reac! tivation "DLEPR# test that indicates the degree of sensitisation to intergranular corrosion[ The second one is the slow strain rate test "SSRT# that enables us to relate the degree of sensitisation to stress corrosion cracking[ A metallurgical study is also performed on two duplex stainless steels of grade UNS S20792[ Sensitisation is carried out by several heat treatments at 564>C or 899>C[ The in~uence of s!phase is clearly shown on the mechanical properties and the corrosion resistance[ Þ 0888 Elsevier Science Ltd[ All rights reserved[ Keywords] Intergranular corrosion^ Stress corrosion^ Duplex stainless steel

0[ Introduction Duplex "austeno!ferritic# stainless steels are used for a lot of applications because of their good mechanical properties and pitting resistance[ However\ some problems can appear due to microstructural modi_cations provided by welding or heat treat! ment[ The heat a}ected zone produced by welding greatly a}ects the resistance of the material[ The heat treatments lead to the precipitation of various compounds like chromium carbides\ nitrides and some other intermetallic phases like s!phase[ These microstructural modi_cations can have a detrimental a}ect on the behaviour of the steel[ There are very few studies about the e}ect of s!phase on duplex stainless steels[ Li

 Corresponding author[ 9909!827X:88:, ! see front matter Þ 0888 Elsevier Science Ltd[ All rights reserved[ PII] S 9 9 0 9 ! 8 2 7 X " 8 8 # 9 9 9 9 8 ! 7

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et al[ ð0Ł have shown that s!phase nucleates at the austenite "g#!ferrite "a# interface and grows through the ferrite in a cellular morphology[ Moreover\ the chemical composition of the precipitates is modi_ed as they grow] there is an increase in molybdenum but a decrease in chromium[ The appearance of the brittle s!phase induces the reduction of ductility[ The results of Boppert et al[ ð1Ł are in agreement with the previous remarks since they observed a small quantity of s!phase which appeared on the ferriteÐaustenite interface after only 2 min at 749>C[ The toughness is reduced when the material is heat treated between 799>C and 0999>C[ Redja(mia ð2Ł has made a study on the s!phase precipitates[ During the ferrite!to!austenite transformation\ the ferrite is enriched in chromium and molybdenum and becomes a preferential site for s!phase formation[ The chromium!depleted areas\ due to the formation of these precipitates\ lead to a decrease in corrosion resistance[ The purpose of this paper is to examine the in~uence of s!phase on the mechanical properties\ the intergranular corrosion and stress corrosion cracking "SCC# of two duplex stainless steels[

1[ Experimental method 1[0[ Materials and heat treatments Two austeno!ferritic "duplex# stainless steels\ type UNS S20792\ are studied in this investigation[ The chemical composition is given in Table 0[ All the specimens are water!quenched for 19 min at 0099>C "T9# in order to hom! ogenise the structures[ Then\ several heat treatments are performed to simulate the di}erent sensitisations] 0 h at 564>C\ 1 h at 564>C\ 09 h at 564>C and 3 h at 899>C[ The heat treatments at 564>C promote the formation of chromium carbides or nitrides\ as can be seen on the timeÐtemperature!transformation diagram "Figure 0#[ For 09 h at 564>C and 3 h at 899>C\ s!phase "FeÐCrÐMo# can also precipitate[ Grosbeck reagent is used to show the precipitates in the structure[ It colours the phases as follows] ferrite*light\ austenite*white and precipitates*dark[ The chemical composition of the precipitates\ given by energy dispersed spec! troscopy "EDS# analysis\ will be able to identify them[

Table 0 Chemical composition "wt[)# of the duplex stainless steels UNS S20792 investigated[ Steel C

Si

Mn

P

S

Cr

Ni

Mo

Cu

N

D0 D1

9[30 9[286

0[5 0[073

9[914 9[912

³9[90 9[9994

11 11[4

4[4 4[23

2[0 2[10

9[14 9[056

9[02 9[06

9[910 9[91

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Fig[ 0[ TimeÐtemperature!transformation diagram for duplex stainless steel UNS S20792 "from Desestret et al[ ð02Ł#[

1[1[ Inter`ranular corrosion test The intergranular corrosion resistance can be reduced by the presence of chromium depleted areas[ In order to relate the degree of sensitisation "DOS# to intergranular corrosion\ the DLEPR "Double Loop Electrochemical Potentiodynamic Reac! tivation# test is used ð3Ł[ It consists of polarising the steel from the corroding potential to a potential in the passive area "Ep#[ Then the scanning direction is reversed to the corroding potential at the same rate[ This reactivation leads to the preferential breakdown of the passive _lm on chromium!depleted areas due to precipitation[ The current is measured[ The DOS is then given by the ratio\ Ir:Ia\ with Ir being the maximum current recorded during reactivation and Ia the peak current of the _rst loop[ The higher the ratio\ the higher the material sensitisation[ This DLEPR test was _rst used on austenitic stainless steels by Majidi and Streicher ð4Ł[ The solution used was 9[4 M H1SO3¦9[90 M KSCN[ They consider that a sensitised material should present a current ratio\ Ir:Ia\ higher than 9[94[ Some other studies have been made on the DLEPR test ð5Ð7Ł[ In 0883\ Otero et al[ ð8Ł show that there is a correlation between the s!phase content of a duplex stainless steel and the reactivation loop[ Nevertheless\ some limitations of the DLEPR test appear] the size of the precipitates could lead to few chromium!depleted areas and\ therefore\ to a low current ratio while the material is very sensitised[ In the present study\ the test is performed according to the recommendations of Majidi and Streicher ð4Ł[ The standard solution is modi_ed to suit duplex stainless steels and becomes] 1 M H1SO3¦9[90 M KSCN¦9[4 M NaCl at 2920>C[ The other experimental conditions are as follows] , specimen "0 cm1# polished with 599!grit paper , Ep¦199 mV vs[ SCE

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, scan rate099 mV min−0 This DLEPR test is not yet normalised ð09Ł[ Therefore\ the results need to be validated by two standard tests which are] ASTM A151"A# oxalic acid etch test\ ASTM A652"Y# immersion test[ The oxalic acid etch test consists of an electrochemical etching in a 09) solution of oxalic acid at a current of 0 A cm−1 for 0[4 min[ The etch structures obtained are classi_ed into three types] , STEP] no ditches at grain boundaries\ , DUAL] some ditches at grain boundaries\ , DITCH] one grain or more completely surrounded by ditches[ A sensitised material showing a ditch structure should present a current ratio of Ir:Ia×09−1 during the DLEPR test ð00\ 01Ł[ The second standard test is the immersion test\ usually applied to ferritic stainless steels[ The DOS is given by the loss of weight due to the dissolution of chromium depleted areas and is expressed as the rate of penetration "mm per year#] MPY76259×

W \ A×t×d

where Wloss of weight "g#^ timmersion time "61 h# Aspecimen area "cm1# and dsteel density "6[5 g cm−2#[

1[2[ Stress corrosion crackin` test The slow strain rate technique is employed[ As the mechanical properties are modi_ed by the presence of s!phase ð0Ł\ in order to relate the DOS to SCC\ it is important to get rid of the purely mechanical modi_cations[ Therefore\ the DOS is given by the ratio] SSCC 0−

oRSCC oRinert

\

where oRSCC is the fracture strain in the corrosive environment and oRinert is the fracture strain in the inert environment[ The SCC tests are performed in MgCl1 boiling at 006>C "29) wt[# or in the same solution used for the DLEPR tests at 29>C "1 M H1SO3¦9[90 M KSCN¦9[4 M NaCl#[ The reference mechanical tests are performed in silicon oil at the same tem! peratures "29>C or 006>C#[ The strain rate is about 09−5 s−0[ The specimens are

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Fig[ 1[ Specimen used for slow strain rate tests[

cylindrical "Figure 1# and polished with diamond paste "9[14 mm#[ A scanning electron microscopy "SEM# is used to ascertain the mode of rupture[

2[ Results and discussion 2[0[ Metallo`raphic etchin`s The micrographs of the structures obtained after etching steel D0 with Grosbeck reagent are represented in Fig[ 2 for the water quenched specimen\ and those treated for 09 h at 564>C and for 3 h at 899>C[ The precipitation starts after 1 h at 564>C[ There is a coarsening of precipitates after 09 h at 564>C[ For the specimen treated for 3 h at 899>C\ a lot of precipitates "about 14)#\ nucleates at the a:g or a:a interfaces and grows through the adjacent ferrite[ To identify the precipitates\ an EDS analysis is performed[ This technique cannot detect nitrogen and carbon so that chromium carbides or nitrides are not identi_ed[ Moreover\ the size of the precipitates is very low[ However\ from the literature ð02Ł\ carbides and nitrides should be present for the lower temperature treatment at 564>C[ The chemical composition of the precipitates for heat treatments\ 09 h:564>C and 3 h:899>C\ is given in Table 1[ As can be seen\ they are rich in chromium and mainly molybdenum\ a composition similar to that of the s!phase[ An evolution can also be observed in the s!phase composition for the duration of the treatment[ These results are in accordance with the investigation of Li et al[ ð0Ł[ The EDS chromium pro_le shows that the formation of s!phase leads to chromium depleted areas "Figure 3# at the s:a and s:g interfaces[ For steel D1\ the structures are similar[ Nevertheless\ the precipitation is less important than for steel D0^ this can be due to the di}erence in composition of each

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Fig[ 2[ Structures of duplex stainless steel D0 after etching with Grosbeck reagent^ ferrite appears in light\ austenite in white\ precipitates in dark^ "a# water!quenched\ "b# 09 h at 564>C\ "c# 3 h at 899>C[

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Table 1 Chemical analysis "EDS# of the di}erent phases "austenite\ ferrite and precipitate# in the two duplex stainless steels[ Steel

Heat treatment

Phase

) Cr

) Ni

) Mo

) Si

) Fe

D0

water!quenched

austenite ferrite precipitate precipitate

19[1 12[91 13[6 18[3

5[07 2[52 2[82 1[81

1[08 2[30 8[05 7[26

9[276 9[381 9[892 9[556

52[15 50[31 59[95 45[26

austenite ferrite precipitate precipitate

08[3 19[6 11[8 18[1

5[95 4[10 2[85 1[87

1[92 1[7 8[65 7[14

9[243 9[241 0[929 9[545

51[81 51[32 48[47 46[75

09 h:564>C 3 h:899>C D1

water!quenched 09 h:564>C 3 h:899>C

Fig[ 3[ Chromium pro_le at interface a:s:g for steel D0 heat treated 09 h at 564>C "EDS analysis# showing chromium depleted areas due to the precipitation of s!phase[

steel and\ in particular\ the nitrogen content[ The in~uence of this element has been widely studied ð03Ð05Ł[ The nitrides formation\ induced by nitrogen\ retards the precipitation of chromium carbides[ Moreover\ nitrides\ which are intragranular precipitates\ do not lead to intergranular chromium depletion ð01Ł[ 2[1[ Inter`ranular corrosion The results of the DLEPR tests and the two standard tests are given in Table 2[ A step structure is obtained for water quenched specimens[ The immersion test provides rather low values of MPY "0 and 9[2 mm per year\ respectively for D0 and D1#[ The Ir:Ia ratios are in accordance with the two previous tests since they are quite low[ After 0 h at 564>C\ steel D1 preserves a step structure while steel D0 has a dual structure[ This di}erence\ already shown in the metallographic study\ can come from the nitrogen content of both alloys since nitrogen is known to retard precipitation[

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Table 2 Results of the intergranular corrosion tests[ Steel

Heat treatment

DLEPR "Ir:Ia#

Oxalic acid etch structure

Immersion test MPY "mm per year#

D0

water!quenched 0 h:564>C 1 h:564>C 09 h:564>C 3 h:899>C

09−3 5×09−2 5×09−1 11×09−1 3×09−1

STEP DUAL DITCH DITCH DITCH

0 0[1 0[5 1[5 0[5

D1

water!quenched 0 h:564>C 1 h:564>C 09 h:564>C 3 h:899>C

9 2×09−3 4×09−3 8×09−1 29×09−1

STEP STEP DUAL DITCH DITCH

9[2 9[3 9[7 2[7 not done

The immersion test provides low values of MPY "0[1 and 9[3 mm per year#[ The observation of the surface after the test shows that there is a beginning of grain boundary dissolution[ The Ir:Ia ratios are in accordance with the standard tests[ After 1 h at 564>C\ there are a lot of precipitates "probably chromium carbides or nitrides# in steel D0\ the structure being ditched[ The Ir:Ia ratio and the MPY value increase[ For steel D1\ the structure is only dual and the values of Ir:Ia and MPY are lower than those for steel D0[ The specimens treated for 09 h at 564>C both have a ditch structure ðFig[ 4"a#Ł and the MPY values are quite high "1[5 and 2[7 mm per year#[ There is an important dissolution of chromium depleted areas\ leading to a high Ir:Ia ratio "11×09−1 and 8×09−1# as seen on the curve ðFig[ 4"b#Ł[ A small dissolution of ferrite can also be observed after the DLEPR test ðFig[ 4"c#Ł[ At least\ for specimens treated for 3 h at 899>C\ the Ir:Ia ratio for steel D0 is lower than the one obtained for the specimen treated for 09 h at 564>C[ The metallurgical study has shown that\ for this material\ a high quantity of ferrite is transformed into s!phase[ However\ the DLEPR test leads to the dissolution of chromium depleted areas but can also dissolve a part of ferrite[ Since there is less ferrite after 3 h at 899>C\ the Ir:Ia ratio is lower[ For steel D1\ the ratio is higher because there is less s!phase than in steel D0 and therefore more ferrite[ These results show the in~uence of s!phase on the intergranular corrosion resistance of duplex stainless steels[ 2[2[ In~uence of s!phase on mechanical properties The results of the mechanical tests in inert environment "silicon oil at 29>C or 006>C# and SCC tests "MgCl1 at 006>C or DLEPR solution at 29>C# are given in Table 3[ The typical stressÐstrain curve for steel D1 is given in Fig[ 5"a\b# for the mechanical tests\ and Fig[ 5"c\d# for the SCC tests[

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Fig[ 4[ Intergranular corrosion test for steel D0 treated 09 h at 564>C^ "a# structure ditch after oxalic acid etching\ "b# DLEPR curve\ "c# structure after the DLEPR test showing intergranular attack and some dissolution of ferrite[

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Table 3 Results of slow strain rate tests\ in inert environment "silicon oil# and in corrosive environment ] 29>C is for DLEPR solution and 006>C is for MgCl1[ Steel

Heat treatment

Temperature ">C#

oR inert ")#

oR SCC ")#

DOS to SCC ")#

D0

water!quenched 0 h:564>C 1 h:564>C 09 h:564>C 3 h:899>C

006 006 006 006 006

19[2 10[8 19[3 04[4 6[8

09[0 7[2 8[5 6[5 1

49[1 51[0 41[8 49[8 63[5

D1

water!quenched

29 006 29 006 29 006 29 006 29 006

20[6 16[3 18 13[0 15[8 13[5 16[1 19[3 01[4 7[8

07[0 07 05[4 8[7 04[5 04[0 03 04 2[2 3[5

31[8 23[2 32[0 48[2 31 27[5 37[4 15[4 62[5 37[2

0 h:564>C 1 h:564>C 09 h:564>C 3 h:899>C

The water!quenched specimens present high mechanical properties " for example\ for steel D1\ the yield stress is about 599 MPa#\ which con_rms the good resistance of duplex stainless steels[ Heat treatments of sensitisation lead to a loss of mechanical properties[ Ductility for steel D1 treated for 0 h at 564>C reaches 18) "tested at 29>C# and shows a drop of 16[1) after a 09 h treatment at 564>C[ This loss can be attributed to the increase of s!phase content\ which is known to be hard but brittle ð5Ł[ The loss of the mechanical properties is better shown for the steel treated for 3 h at 899>C\ where s!phase is very present[ The material exhibits very poor ductility "01[4) for steel D1 tested at 29>C#[ The e}ect of s!phase on the mechanical properties of duplex stainless steels is therefore signi_cant[ Thus\ it is obvious that purely mechanical tests are necessary to evaluate the part of SCC[ At least\ the temperature of the testing solution is important since the properties are generally lower after testing at 006>C than at 29>C[ 2[3[ In~uence of s!phase on SCC The degrees of sensitisation "DOS# to SCC are expressed in Table 3 and Fig[ 6[ An increase in the DOS for the materials treated for 0 h at 564>C can be observed[ Then it rises again after a 3 h treatment at 899>C\ to reach quite high values "63)#[ The presence of s!phase seems to have a strong in~uence on the behaviour front of SCC[ The aspects of the fractured surfaces are illustrated in Figs[ 7Ð09\ respectively\ for

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Fig[ 5[ "a\b# StressÐstrain curve from SSRT in inert environment for steel D1[ "c\d# StressÐstrain curve from SSRT in corrosive environment for steel D1[

water!quenched specimens with a 09 h treatment at 564>C and a 3 h treatment at 899>C in both solutions[ In MgCl1 "pH¼4#\ for the water!quenched specimens\ the fractured surface is transgranular in both phases\ so that austenite and ferrite take part in the _nal fracture[ Then\ a transition appears for a 0 h treatment at 564>C\ which corresponds to a rise in the DOS[ The cracking becomes intergranular around austenite[ This is emphasised for a 09 h treatment^ then\ only ferrite is responsible for the _nal fracture[ For the materials treated for 3 h at 899>C\ the fractured surface is brittle[ The e}ect of s!phase on the mechanism of stress corrosion cracking is therefore signi_cant[ In the DLEPR solution "pH³9#\ on water quenched material\ a complete dis! solution of ferrite can be observed on fracture surfaces and crack sections with a very blunt aspect of crack tip[ The dissolution\ which is more important when precipitates appear "presence of chromium depleted zones#\ seems to be the main SCC mechanism\ even if the hydrogen e}ect can be present[ Nevertheless\ since the s!phase is formed by replacing ferrite\ when there are a lot of precipitates "in the case of 3 h at 899>C#\ the dissolution process is more localised[ A summary of the in~uence of s!phase on SCC is illustrated in Fig[ 00[

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Fig[ 6[ Degree of sensitisation to stress corrosion cracking vs[ heat treatment for the two duplex stainless steels\ after SSRT in MgCl1 boiling at 006>C[

At least\ it appears that steel D1 is more resistant than steel D0 "higher mechanical properties\ lower DOS\ fewer precipitations#[ This is probably due to the di}erences in the chemical composition since steel 1 contains more nitrogen than steel D0[

3[ Conclusion The in~uence of s!phase on the mechanical properties and the corrosion resistance of two duplex stainless steels "type UNS S20792# have been investigated[ The di}erent tests performed lead to the following conclusions] 0[ the heat treatment at 564>C or 899>C leads to the formation of s!phase\ providing chromium depleted areas^ 1[ there is an evolution in the chemical composition of s!phase "Cr\ Mo# with the time and temperature of the treatment^ 2[ the harmful e}ect of s!phase on the intergranular corrosion is clearly shown by

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Fig[ 7[ Details of surface rupture "SEM# for steel D1 water!quenched^ "a# in MgCl1 showing transgranular path\ "b# in DLEPR solution showing dissolution of ferrite and ductility of austenite[

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Fig[ 8[ Details of surface rupture "SEM# for steel D1 heat treated 09 h at 564>C^ "a# in MgCl1 showing some intergranular path around austenite\ "b# in DLEPR solution showing an important dissolution of ferrite[

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Fig[ 09[ Details of surface rupture "SEM# for steel D1 heat treated 3 h at 899>C^ "a# in MgCl1 showing a brittle aspect of the rupture\ "b# in DLEPR solution showing dissolution of ferrite[

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Fig[ 00[ In~uence of s!phase on the stress corrosion cracking[

the results of the Double Loop Electrochemical Potentiodynamic Reactivation "DLEPR# tests^ 3[ the ductility is reduced by the precipitation of s!phase^ 4[ the in~uence of s!phase on SCC is signi_cant] *in MgCl1\ without precipitates\ both phases " ferrite and austenite# take part in the _nal rupture "transgranular cracking#\ while ferrite is the only responsible factor when there are precipitates "intergranular cracking around austenite#^ *in the DLEPR solution\ the dissolution of ferrite and of chromium depleted zones is the main mechanism of SCC[

References ð0Ł J[ Li\ T[ Wu\ Y[ Riquier\ Mat[ Sci[ + Eng[ A063 "0883# 038Ð045[ ð1Ł C[ Boppert\ A[ Schram\ in] Conference] Duplex Stainless Steel |83\ Glasgow\ Scotland\ 02Ð05 Nov! ember 0883[ ð2Ł A[ Redjaimia\ Thesis[ Institut National Polytechnique de Lorraine\ France\ 0880[ ð3Ł V[ Cihal\ A[ Desestret\ M[ Froment\ G[H[ Wagner\ in] Proceedings of the 4th European Conference of Corrosion\ Paris\ France\ 13Ð17 September 0862\ pp[ 138Ð143[ ð4Ł A[P[ Majidi\ M[A[ Streicher\ Corrosion 39 "0873# 473Ð481[ ð5Ł S[J[ Goodwin\ B[ Quayle\ F[W[ Noble\ Corrosion 32 "0876# 632Ð636[ ð6Ł G[S[ Was\ V[B[ Rajan\ Corrosion 32 "0876# 465Ð468[ ð7Ł M[ Verneau\ C[ Lojewski\ J[ Charles\ in] Proceedings of the Conference] Duplex Stainless Steel |80[ Editions de Physique\ 0881\ pp[ 752Ð769[ ð8Ł E[ Otero\ C[ Merino\ C[ Fosca\ F[ Fernandez\ in] Conference] Duplex Stainless Steel |83\ Glasgow\ Scotland\ 02Ð05 November 0883[ ð09Ł C[ Cabrillac\ V[ Cihal\ Corrosion Localisee[ Editions de Physique\ 0883\ pp[ 376Ð387[

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ð00Ł N[ Lopez\ Thesis\ University of Bordeaux\ France\ 0886[ ð01Ł N[ Lopez\ M[ Cid\ M[ Puiggali\ I[ Azkarate\ A[ Pelayo\ Mat[ Sci[ + Eng[ A118 "0886# 012Ð017[ ð02Ł A[ Desestret\ J[ Charles\ Les Aciers Inoxydables[ Editions de Physiques\ Les Ulis\ France\ 0889\ pp[ 520Ð567[ ð03Ł N[ Sridhar\ J[ Kolts\ Corrosion 32 "0876# 535Ð540[ ð04Ł T[A[ Mozhi\ W[A[T[ Clark\ K[ Nishimak\ W[B[ Johnson\ D[D[ MacDonald\ Corrosion 30 "0874# 444Ð448[ ð05Ł H[S[ Betrabet\ K[ Nishimoto\ B[E[ Wilde\ W[A[T[ Clark\ Corrosion 32 "0876# 66Ð73[